Argument values in `bmesh.ops.similar_verts()`
Check the source where docs are lacking.

Similarly to this answer re bmesh.ops.delete look at source of bmesh/intern/bmesh_operators.h

On compare

enum {
    SIM_CMP_EQ = 0,
    SIM_CMP_GT,
    SIM_CMP_LT
};


The int enum values for equality is 0, greater than 1, and less than 2.

On types

/* similar vertex selection slot values */
enum {
    SIMVERT_NORMAL = 0,
    SIMVERT_FACE,
    SIMVERT_VGROUP,
    SIMVERT_EDGE
};


On types it appears to check, 0 checks normals, 1 linked faces, 2 vertex group, and 3 linked edges.

Note

The bmesh similar operators appear to have been removed from 2.8